The present invention relates to a method of manufacturing a semiconductor device, particularly, to a method of manufacturing a semiconductor device having an MIS structure.
In accordance with miniaturization of a silicon semiconductor integrated circuit, an MIS (Metal Insulator Semiconductor) type semiconductor device is being made finer and finer. In recent years, the minimum size of the MIS type semiconductor device is approaching 0.1 .mu.m. In the semiconductor device sized at about 0.1 .mu.m, the gate oxide film is required to have a thickness of 2.5 nm in order to allow the device to maintain a satisfactory performance.
However, if the gate oxide film has a thickness smaller than 4 nm, a leak current through the gate oxide film is rapidly increased by a direct tunnel mechanism. The increased leak current lowers the reliability of the MIS type semiconductor device and increases the power consumption. These difficulties are unavoidable as far as a silicon oxide film is used as a gate insulating film.
In order to avoid these difficulties, vigorous studies are being made in an attempt to use a material having a dielectric constant higher than that of silicon oxide for forming the gate insulating film. Particularly, silicon nitride, which is highly compatible with the conventional silicon process, is expected to provide the most hopeful material for forming a gate insulating film in near future.
The method of forming a silicon nitride film includes mainly a thermal nitridation process in which a silicon substrate is nitrided under an ammonia atmosphere and a CVD (Chemical Vapor Deposition) process in which a nitride film is deposited on a silicon substrate. The former process (thermal nitridation process) makes it possible to form a nitride film small in trapping and low in interfacial energy level. However, the nitride film thickness is saturated at about 3 nm. On the other hand, the latter method (CVD method) certainly permits forming a nitride film of a sufficiently large thickness. However, it is difficult to obtain a high quality silicon nitride film.
As pointed out above, it is impossible to obtain a silicon nitride film having a sufficiently large thickness and a high film quality by any of the thermal nitridation process of a silicon substrate surface and the CVD method.